An example of various types of control systems for a three-phase motor is disclosed in Japanese Patent Publication No. 3312472.
A control system disclosed in the Patent Publication superimposes, on an output signal from an inverter, a high-frequency voltage signal oscillating in the positive and negative directions in an estimated d-axis of a salient three-phase motor, and estimates the rotation angle of the motor based on a high-frequency current signal actually created in the motor.
The salient structure (salient characteristic) of the three-phase motor means a characteristic in which a magnetic resistance created in the d-axis of the three-phase motor is higher than that created in a q-axis thereof. In other words, the salient structure (salient characteristic) of the three-phase motor means a characteristic in which inductance in the d-axis of the three-phase motor is lower than that in the q-axis thereof.